//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012- OpenVPN Inc.
//
//    SPDX-License-Identifier: MPL-2.0 OR AGPL-3.0-only WITH openvpn3-openssl-exception
//

#ifndef OPENVPN_CLIENT_ASYNC_RESOLVE_ASIO_H
#define OPENVPN_CLIENT_ASYNC_RESOLVE_ASIO_H

#include <openvpn/io/io.hpp>
#include <openvpn/asio/asiowork.hpp>

#include <openvpn/common/bigmutex.hpp>
#include <openvpn/common/rc.hpp>
#include <openvpn/common/hostport.hpp>


namespace openvpn {
template <typename RESOLVER_TYPE>
class AsyncResolvable
{
  private:
    typedef RCPtr<AsyncResolvable> Ptr;

    class ResolveThread : public RC<thread_safe_refcount>
    {
        friend class AsyncResolvable<RESOLVER_TYPE>;

      private:
        typedef RCPtr<ResolveThread> Ptr;

        openvpn_io::io_context &io_context;
        AsyncResolvable<RESOLVER_TYPE> *parent;
        std::atomic<bool> detached{false};

        ResolveThread(openvpn_io::io_context &io_context_arg,
                      AsyncResolvable<RESOLVER_TYPE> *parent_arg,
                      const std::string &host,
                      const std::string &port)
            : io_context(io_context_arg),
              parent(parent_arg)
        {
            std::thread t([self = Ptr(this), host, port]()
                          {
	  openvpn_io::io_context io_context(1);
	  openvpn_io::error_code error;
	  RESOLVER_TYPE resolver(io_context);
	  typename RESOLVER_TYPE::results_type results;

	  results = resolver.resolve(host, port, error);
	  if (!self->is_detached())
	  {
	    self->post_callback(results, error);
	  } });
            // detach the thread so that the client won't need to wait for
            // it to join.
            t.detach();
        }

        void detach()
        {
            detached.store(true, std::memory_order_relaxed);
            parent = nullptr;
        }

        bool is_detached() const
        {
            return detached.load(std::memory_order_relaxed);
        }

        void post_callback(typename RESOLVER_TYPE::results_type results,
                           openvpn_io::error_code error)
        {
            openvpn_io::post(io_context, [self = Ptr(this), results, error]()
                             {
	  auto parent = self->parent;
	  if (!self->is_detached() && parent)
	  {
	    self->detach();
	    OPENVPN_ASYNC_HANDLER;
	    parent->resolve_callback(error, results);
	  } });
        }
    };

    openvpn_io::io_context &io_context;
    std::unique_ptr<AsioWork> asio_work;
    typename ResolveThread::Ptr resolve_thread;

  public:
    using resolver_type = RESOLVER_TYPE;
    using results_type = typename RESOLVER_TYPE::results_type;

    AsyncResolvable(openvpn_io::io_context &io_context_arg)
        : io_context(io_context_arg)
    {
    }

    virtual ~AsyncResolvable()
    {
        async_resolve_cancel();
    }

    virtual void resolve_callback(const openvpn_io::error_code &error,
                                  results_type results)
        = 0;

    // mimic the asynchronous DNS resolution by performing a
    // synchronous one in a detached thread.
    //
    // This strategy has the advantage of allowing the core to
    // stop/exit without waiting for the getaddrinfo() (used
    // internally) to terminate.
    // Note: getaddrinfo() is non-interruptible by design.
    //
    // In other words, we are re-creating exactly what ASIO would
    // normally do in case of async_resolve(), with the difference
    // that here we have control over the resolving thread and we
    // can easily detach it. Deatching the internal thread created
    // by ASIO would not be feasible as it is not exposed.
    virtual void async_resolve_name(const std::string &host, const std::string &port)
    {
        resolve_thread.reset(new ResolveThread(io_context, this, host, port));
    }

    // there might be nothing else in the main io_context queue
    // right now, therefore we use AsioWork to prevent the loop
    // from exiting while we perform the DNS resolution in the
    // detached thread.
    void async_resolve_lock()
    {
        asio_work.reset(new AsioWork(io_context));
    }

    // to be called by the child class when the core wants to stop
    // and we don't need to wait for the detached thread any longer.
    // It simulates a resolve abort
    void async_resolve_cancel()
    {
        if (resolve_thread)
        {
            resolve_thread->detach();
            resolve_thread.reset();
        }

        asio_work.reset();
    }
};
} // namespace openvpn

#endif /* OPENVPN_CLIENT_ASYNC_RESOLVE_ASIO_H */
